1. Field of the Invention
In FM receivers, it is a well known fact that the level of the noise emanating from the discriminator is inversely proportional to the strength, or quality, of the signal being received by the receiver and, hence, the level of the noise is a valid indication of the strength, or quality, of the signal. The major problem determining the amplitude of the noise signal emanating from the discriminator is in measuring the noise with no other signals present. If any audio or other interference is present when the noise is being measured, the measurement will be too high and will be a false indication of the true strength or quality of the signal being received.
2. Description of the Prior Art
Typically, in the prior art devices utilizing a signal quality indicator, the circuitry monitored the entire audio output of the discriminator. These systems can obtain a fairly accurate reading of the noise level if the audio contains periodic pauses. However, if there are only minor pauses in the audio, this prior art system is highly inaccurate. For example, when continuous tones or data are received because there are no pauses or valleys, the prior art systems can not obtain a reading of the noise level. A prior art system of this type is described in U.S. Pat. No. 3,729,681, entitled "Receiver Selecting Arrangement," issued Apr. 24, 1973.
In another type of signal quality indicator, circuitry is provided to monitor the output of the discriminator above the audio spectrum. Circuitry utilizing this type of signal quality indicator is described in U.S. Pat. No. 3,769,592, entitled "Squelch Circuit with Time Delay Variable in Accordance with Strength of Received Signal," issued Oct. 30, 1973. This type of signal quality indicator is very accurate and reliable but is not useful in a voting system or receiver selecting system, such as that described in the U.S. Pat. No. 3,729,681 (cited above) since telephone lines, which are utilized to carry the audio signals from the receiver to a central selecting site, have a limited bandpass of approximately 300 to 3,000 cycles. Therefore, the discriminator output above the audio spectrum is removed by telephone lines and not available as an indicator of signal quality.